1. Field of the Invention
This invention relates to an Single Side Band hereinafter referred to as a (SSB) transmitter, and in particular relates to improvements to prevent a decrease in suppression ratio of a carrier wave and a deterioration of a sound signal that occur due to a temperature characteristic of a band path filter (hereinafter referred to as a BPF) which extracts only one of the sideband signals after an amplitude modulation of a carrier frequency signal by an input signal.
2. Related Art
Since the SSB communication system is to perform its communication function using either one of the sideband signals and because of a benefit such that an occupied frequency is almost half of a DSB (Double Side Band) it allows an effective use of wave and a saving of electric power for transmission, it forms the main currents in AM wireless radio telephone communication system.
Conventionally, an SSB transmitter has a basic composition as shown in FIG. 6.
First, a sound input signal fs from a microphone 1 is amplified by an amplifier 2 and inputted to a balanced modulator 3, and the balanced modulator 3 amplitude-modulates a carrier frequency signal (frequency: f01), which is inputted from a local oscillator 4, by the sound input signal fs.
From the balanced modulator 3, two sideband signals (sound modulated signals) with frequency bands of (f01+fs) and (f01xe2x88x92fs) are obtained, and in this transmitter the lower sideband (LSB) signal (f01xe2x88x92fs) is extracted and outputted to a mixer 6 by a BPF 5.
At the mixer 6, two signals with frequency bands of [f02+(f01xe2x88x92fs)] and [f02xe2x88x92(f01xe2x88x92fs)] are obtained by mixing a signal (frequency: f02) of a VHF band of a local oscillator 7 and said LSB signal (f01xe2x88x92fs), and in this transmitter the LSB signal [f02xe2x88x92(f01xe2x88x92fs)] that corresponds to a difference component is extracted by a BPF 8.
Said LSB signal [f02xe2x88x92(f01xe2x88x92fs)] is inputted to a mixer 9, and at the mixer 9, to obtain a desired multi-channel emission frequency, a selective channel frequency signal (fLxcx9cfH) by a PLL synthesizer 10 and the LSB signal are mixed and a difference component (fLxcx9cfH)xe2x88x92[f02xe2x88x92(f01xe2x88x92fs)] is extracted.
In this case, the phase of the difference component signal is inverted to make it an upper sideband (USB) signal.
And, the USB signal is amplified by an excitation amplifier/power amplifier 11, outputted to an antenna 12 and emission-transmitted from the antenna 12.
Incidentally, in this SSB transmitter, in which LSB signals are extracted at both the BPF 5 and BPF 8, which sideband signal to extract differs by the specification of SSB transmitter.
The BPF 5 is a filter to suppress carrier frequency signals and extract only one of the sideband signals, and as the provision of a sharp cut-off characteristic, a low insertion loss and an excellent temperature characteristic are required, generally a crystal filter is used.
Aiming at said temperature characteristic, the crystal filter has an excellent temperature characteristic, but the central frequency of a normal passband presents a change rate of some (Hz/xc2x0 C.) by temperature changes.
Also, though sometimes a ceramic filter is used to save parts costs, but said change rate is considerable compared to the crystal filter.
And, the suppression ratio is set to 40 dB and larger in the SSB communication system, but in view of the effect of the temperature characteristic of said BPF 5 on its function, supposing the frequency f01 of the carrier frequency signal, the central frequency cf of BPF 5 and that FIG. 7(A) is a normal state of extraction, when the central frequency cf deviates towards the direction of FIG. 7(B), the suppression ratio increases, but since the passband shifts towards the low band side, a trend that the low band side of sound signal attenuates occurs, and when the central frequency cf of BPF 5 deviates towards FIG. 7(C) on the contrary, there is a possibility that the suppression ratio decreases below a specific value and a trend that the higher band side of sound signal attenuates occurs.
That is, there is a problem that, depending on the temperature characteristic of BPF 5, suppression of a carrier wave may become insufficient and the sound may be deteriorated.
To cope with this problem, conventionally efforts were made with measures to improve the temperature and cut-off characteristics of BPF 5 as much as possible, but naturally, since the room for improvements related to said characteristics of BPF was limited, it was not possible to take any sufficient measures.
Thus, in consideration of the above problem, this invention is made with an object of supplying an SSB transmitter that, even if the central frequency of the BPF, which is intended to extract only one sideband of modulated sound signals while suppressing carrier waves, changes by temperature, can always accommodation-compensate it and maintain a normal state of extraction.
A first embodiment relates to an SSB transmitter for amplitude-modulating a carrier frequency signal by first oscillation means by an input signal, extracting one of the sideband signals from the modulated signal by a first filter, mixing the sideband signal with a local oscillation frequency signal of second oscillation means, and extracting out of a sum component signal and a difference component signal obtained by said mixing the difference component signal by a second filter so as to obtain a radio frequency signal with a prescribed frequency, comprising: temperature detection means for detecting a temperature of said first filter; memory means for storing changes in central frequency of a passband due to temperature changes from a reference temperature at said first filter; and control means for changing the frequency of output signals of said first and second oscillation means by xcex94f when a change in central frequency of the passband of said first filter obtained by said memory means is xcex94f based on a temperature information detected by said temperature detection means, while setting a reference frequency of output signals of said first and second oscillation means for a frequency with which a radio frequency signal with said prescribed frequency can be obtained in the passband of said first filter at said reference temperature.
In this embodiment, the memory means previously stores the changes in central frequency due to temperature changes of the first filter, and based on the temperature information obtained from the temperature detection means that detects temperatures of the first filter, the control means detects the change xcex94f in said central frequency corresponding to a temperature change of the first filter, using the memory means.
And, the control means, by adapting the frequency of output signal of the first oscillation means for the passband of the first filter, where the central frequency is changed, by changing it by xcex94f, enables the first filter to normally perform suppression of carrier signals and extraction of sideband signals.
However, in this case a signal which is extracted by a second filter after being mixed with an output signal of the second oscillation means is also changed by xcex94f.
Thus, the control means is designed to also change the output signal of the second oscillation means by xcex94f along with the control of said first oscillation means so as to obtain a normal radio frequency signal with a prescribed frequency.
Additionally, in this embodiment and in a second embodiment described below, the xe2x80x9cradio frequency signalxe2x80x9d means a signal extracted by the second filter, and in the case of mixing with a selected channel frequency signal by a PLL synthesizer and emission-transmission from an antenna, it corresponds to a signal before the mixing.
A second embodiment relates to an SSB transmitter for extracting a sum component signal, while the first embodiment aims at an SSB transmitter for extracting out of a sum component signal and a difference component signal after mixing the difference component signal by a second filter.
And, the temperature detection means, the memory means and control to the first oscillation means by the control means are the same as the first embodiment, while control related to the second oscillation means changes the frequency of output signal by said change to the opposite side. That is, this changes the output signal of the second oscillation means by xe2x88x92xcex94f.
In this embodiment, since the sum component signal side of the sum component signal and the difference component signal obtained by mixing the sideband signal extracted by the first filter with the output signal of the second oscillation means must not contain the change xcex94f, to cancel this, the output signal of the second oscillation means needs to be changed by xe2x88x92xcex94f.
Accordingly, the SSB transmitter of this embodiment too, like the first embodiment, can compensate changes in passband due to temperature changes of the first filter and always obtain a normal radio frequency signal with a prescribed frequency.